It is known that high strength nickel, cobalt or iron based superalloys, for example nickel based superalloys with additional elements such as aluminum and titanium, have their high strength characteristics because of the precipitation hardening effect of the high degree of gamma-prime phase in the material. It is also known that those superalloys are very difficult to weld successfully.
SLM-generated articles have different microstructures compared to conventionally cast material of the same alloy. The high energy beam-material interaction in these processes leads to high cooling rates and very fast solidification during SLM.
As a consequence, segregation of alloying elements and formation of precipitates are reduced. Therefore, due to the rapid cooling inherent to the additive manufacturing processes, few to no gamma-prime precipitates are present in the component made of gamma-prime containing alloys after build-up.
Post-built heat treatments are needed to adjust the microstructure of the part and to reduce/eliminate residual stresses. During such post-built heat treatments the gamma-prime phase precipitates during the first heat-up. But the volume changes associated with this precipitation can lead to significant cracking in the part (e.g. strain age cracking). Currently applied heat treatment sequences applied for SLM-processed gamma-prime strengthened superalloys lead to significant cracking and therefore to rejection of parts.
Using different pre- and post-weld heat treatments is known for joining cast components or parts of components made of gamma-prime (γ′) strengthened superalloys by welding.
U.S. Pat. No. 7,854,064 B2 discloses a process for repair of turbine parts that includes a pre-weld solutioning heat treatment using heating rates between 16-23° C./min in a temperature range between 593-871° C. In one embodiment a slow cooling rate of 0.2-5° C./min from solutioning temperature to below 677° C. is mentioned. In addition, besides the above mentioned pre-weld heat treatment a post weld heat treatment is described using the same heating rate as the pre-weld heat treatment. The process according to this document is applicable to a wide variety of cast and wrought nickel based alloys, for example Waspaloy, IN738, IN792 or IN939. E-beam and tungsten arc welding are mentioned as example processes.
Although the method disclosed in U.S. Pat. No. 7,854,064 B2 has the advantage that turbine components made of Nickel based superalloys could be repaired e.g. welded virtually without the presence of microcracks it has the disadvantage of being time and cost consuming with respect to the described multiple steps of pre-weld and post-weld heat treatment.
The applicant filed a new patent application related to e-beam welding of gamma-prime strengthened superalloys (e.g. IN738LC, MarM247, CM247LC, CMSX-4, MK4HC, MD2) without weld filler recently (not published yet). In contrast to U.S. Pat. No. 7,854,064 this method does not depend on a specific pre-weld heat treatment and thus can be used for repairs as well as for joining new parts. To make the process more efficient a fast heating rate is used in the entire temperature range (rather 1100° C. than 871° C.) close to the final hold temperature, where gamma-prime can precipitate. This method is used only in connection where no other means of crack avoidance exist, i.e. welding processes without weld filler. Using a ductile weld filler could also help avoiding crack formation, however the use of such weld fillers weakens the weld joint.
However, above-mentioned documents cover only joining methods (e.g. welding) and do therefore not cover components entirely made by additive manufacturing, for example by selective laser melting (SLM).